Question about black holes

when a star collapses and becomes a black hole, why does its gravitational force become so much greater? since gravitational force is proportional to mass, not density, why would the black holes suddenly become so much more powerful than the star they once were??

Speaking roughly, the gravitational force does not increase at a fixed distance away. The difference is that you can get much closer to the center once the star has collapsed (Note that burrowing through an intact star wouldn't be the same. The gravitational field would reach its maximum at the surface.).

As the previous responders noted, the gravitational force does not increase. If a star could non-catastrophically collapse into a black hole, its planets would remain in orbit just as if nothing had happened. While what you said about gravitational force being dependent upon mass rather than density is true, the shape of the field is density reliant. Remember that the force is a function of the distance between the centres of the involved masses, not the surfaces, and follows the inverse square law. Stingray's explanation of being able to get much closer to the centre of a black hole than a full star is the correct one.

thanks. i hate to keep pestering about this, but if the gravitational attraction does not increase, then why do black holes start swallowing everything around them. if the planets of the star werent close enough to get sucked in when it was a star, then they would be outside the event horizon and not get sucked in when it becomes a black hole, unless of course they are involved in the catastrophic collapse.

It's quite possible that there are black holes out there with planets around them. They wouldn't be habitable, of course. Even if they had been to start with, the red giant phase or nova explosion would have altered the neighbourhood significantly. I don't know of a gentle way for a star to collapse to BH stature.
If you're thinking of the pictures often shown of a BH in a binary system devouring its companion, I suspect that it's because the companion's expansion due to normal stellar evolution moved its surface closer to the BH. That's about as far as I can go. Someone with more knowledge of the field will have to take over.

As a partial caveat, objects that happen to get very close (much much closer than they ever could when the star was intact) will start to emit gravitational radiation. This carries away energy, which could allow the object to spiral into the hole. But this is a very extreme scenario. If a star could "quietly" collapse into a black hole, the orbits of its planets wouldn't be affected at all.

As a partial caveat, objects that happen to get very close (much much closer than they ever could when the star was intact) will start to emit gravitational radiation. This carries away energy, which could allow the object to spiral into the hole. But this is a very extreme scenario. If a star could "quietly" collapse into a black hole, the orbits of its planets wouldn't be affected at all.

Basically it's space rippling. When you drop a stone into a pond, ripples spread out from the point where the stone hits the water. When something moves through space it does a similar thing, it sends out a wave through space. For most things the ripples are extremely weak. I think I read somewhere that the Earth gives off about 1000W of gravitational radiation. When an entire star collapses into a black hole (or even more powerful, two black holes collide) much more gravitational radiation is given off. Someone in another thread said that if two black holes the mass of the Sun (each) merged 0.59 solar masses of energy would be given off. That's many many times more powerful than a supernova, but all through gravity waves. The forces that would produce would shred any nearby planets to pieces (if you're seen 'Star Trek - Generations' it'd be akin the when the planet gets destoyed at the end, you'd just never see the wave of energy coming!).

jasc15 said:

when a star collapses and becomes a black hole, why does its gravitational force become so much greater?

The gravitational acceleration on the surface of an object is [tex]g = \frac{GM}{R}[/tex]. If you keep M constant (and G is constant anyway) but compress the object to make R smaller, say compressing a star into a black hole, then the gravity field on the surface increases enormously. For instance, g = 9.81 on Earth's surface, and R = 6,400,000 metres. If you compress the Earth into the size of a match box, it'd be a black hole. Then R = 0.05m. That's about a value of R 100,000,000 times smaller, so the surface gravity on the 'surface' would have got 100,000,000 times stronger all because you made the object more compact.

That's just using Newtonian gravity. Relativity would be required to do it properly, but you get the general idea.